The present invention relates to an image forming apparatus wherein a toner image is fixed on a sheet at a nip portion that is formed by rotary bodies which rotate while facing each other.
Many of image forming apparatuses of an electrophotographic technology type have a fixing section that fixes a toner image on a sheet by conveying the sheet that carries a toner image while pressurizing and heating it in a nip portion that is formed by rotary bodies which face each other to rotate, for example, by rollers or by a belt that is trained about plural rollers.
The fixing section constituted in this way sometimes has a so-called winding accident wherein toner on a sheet is melted by heating, and viscosity of the melted toner causes the sheet to stick to the rotary body for heating a toner image, and the sheet is not separated.
To avoid the troubles mentioned above, there is employed widely a technical measure to separate a sheet from a roller by causing a separation claw or a member for separation called a separation sheet to touch the roller.
However, the technology to separate a sheet from a roller by using a member for separation that is like one mentioned above sometimes generates a scratch on an image surface of a print, because it is a technical measure to cause an image surface of a sheet to touch the member for separation.
The scratch of this kind is a main cause to lower greatly a quality of an image where toner sticks to a certain entire area, representatively, a print on which a color image is formed.
Therefore, a technical measure to separate a sheet from a roller by blowing air against a leading edge of the sheet that has just passed through the nip portion is also employed widely, as a measure to separate a sheet from a roller without touching an image surface of the sheet sticking to the rotary body (for example, see Unexamined Japanese Patent Application Publication No. 2004-212954).
In general, a force for separating a sheet from a rotary body (first rotary body) is enhanced more when air to be blown is strengthened more. However, in this case, the sheet thus separated is pressed strongly against another rotary body (second rotary body) that is arranged to face the first rotary body, or is pressed strongly against a guide member provided to be close to the second rotary body.
The phenomenon of this kind makes separation of a sheet from the second roller to be difficult, or, strong touch between the guide member and the image having already been formed on the reverse side causes new occurrence of scratches on the image surface, resulting in a cause of an image quality decline, when images are formed on both sides of a sheet.
Further, for preventing occurrence of the aforesaid troubles, there is a suggestion to arrange air blowing devices to face the first rotary body and the second rotary body (For example, see Unexamined Japanese Patent Application Publication No. 2006-113342).
However, it was found out that behaviors of a leading edge of a sheet become unstable, and dangerousness for occurrence of jamming increases when doing nothing except blowing air against a leading edge of a sheet by arranging air blowing devices to face both of the first and second rotary bodies.
The invention has been achieved in view of the aforesaid circumstances, and its objective is to realize an image forming apparatus wherein nozzles each blowing air against a position established on a circumferential surface of each of the two rotary bodies which face each other to rotate are provided at positions corresponding to the aforesaid two rotary bodies, and timing of blowing air of each nozzle is controlled based on a position of an advancing sheet to separate a sheet from the two rotary bodies surely.